Polyhydroxyalkanoates and film formation therefrom

ABSTRACT

A supported or cast film of a polyhydroxyalkanoate polymer, such as a polyhydroxybutyrate/valerate polymer, can be prepared by applying a layer of molten polymer to a cool (typically 4° to 20° C.) surface not substantially above the glass transition temperature of the polymer, so as to form a solid, glassy film with a high density of nucleation sites; subsequently the temperature of the film so formed is raised, for example to 40° C. or more, towards the optimum temperature for growth of the polymer&#39;s spherulites effectively separating the film formation/nucleation step from the crystallisation step (growth of spherulites around the nucleation sites). Smaller crystallites are formed, and the film has improved barrier properties.

THIS INVENTION relates to polyhydroxyalkanaoates and the production offilms therefrom, which may be free standing or supported.

Films of polymers have many uses. Free standing films of plasticsmaterials have innumerable industrial and consumer applications.Supported films, for example coated paper, are also widely useful,particularly for imparting the qualities of moisture resistance andheat-sealability to the coated support.

In principle, films can be prepared from polymer either by using apreparation of molten polymer or by the use of a solution of polymer ina suitable solvent. In the latter case, it is clearly necessary to matcha polymer with a suitable solvent for it; for polyhydroxyalkanoatepolymers, which have the merit of being biodegradable, a problem existsin that the suitable solvents are generally chlorinated and thereforeenvironmentally detrimental. Attention has therefore diverted toattempting to prepare films of these biodegradable polymers from moltenpreparations of the polymer; and it is to this area that the presentinvention is directed.

Extrusion coating of polymer films from molten polymer has beendeveloped for a number of plastics, in particular low densitypolyethylene. The standard procedure is for molten polyethylene to beextrusion coated onto a chilled roller, known in the art as achill-roll. Low density polyethylene crystallises very quickly, and atreasonably low temperatures, so that it is adequately crystalline beforeit is drawn off the chill-roll, even when the roll is operating at veryhigh line speeds, such as 700 m/min. Polyhydroxyalkanoate polymerscrystallise much more slowly than low density polyethylene at a typicalchill-roll temperature.

The process of polyhydroxyalkanoate crystallisation from the moltenstate involves growth of crystallites which are often spherical regionsof crystallised material (spherulites). A nucleation site for acrystallite may occur in the pure polymer or can be artificiallyintroduced by the use of a nucleating agent. We have found that theoptimum temperature for creating nucleating sites is considerably lowerthan the optimum temperature for growth of the crystallites.

It has now been found that it is possible to adapt a chill-roll processfor the preparation of a polyhydroxy-alkanoate film if the filmformation is allowed to take place on the roll at a relatively lowtemperature, so as to improve or optimise nucleation density andmaintain the film in a glassy state which is of low tackiness andsubsequently the film temperature is raised to enable crystallisation totake place. Acceptable line speeds are practicable.

The film so produced has smaller crystallites than that produced if thechill roll is at higher temperatures. This can have two advantages:firstly, it may lead to improved mechanical properties. Secondly,smaller crystallites can lead to reduced voiding/pinholing at theboundaries between crystallites. This improves the coatings barrierproperties, and is an important feature of films whether free standingor supported for example on paper or board. If the coating tends topinhole or crack, its efficiency is reduced as a barrier medium.Typically paper coatings are between 10 and 25 microns, and hencecrystallites should be smaller than 10 microns. We have devised aprocess which produces crystallites <10 μm and typically <7 μm, mostly3-5 μm.

According to a first aspect of the present invention, there is provideda process for preparing a film of a polyhydroxyalkanoate polymer, theprocess comprising applying a layer of molten polyhydroxyalkanoate to asurface not substantially above the glass transition temperature of thepolymer and for example below 30° C., preferably below 20° C. and morepreferably below 15° C. so as to form a film and subsequently raisingthe temperature of the film so formed to a temperature of 45° to 100° C.and preferably 50° to 95° C. and more preferably 60° to 80° C.

Without wishing to be bound by this explanation, it is believed thatfilm which has a low degree of crystallinity and is well above the glasstransition temperature is unacceptably tacky and that if it has a highdegree of crystallinity it has in general a low extension to break. Itis believed however that the aforesaid process causes the formation of afilm which is not unacceptably tacky with small crystallites and whichhas better mechanical performance.

The polymer after raising the temperature as aforesaid is preferably 40to 70% crystalline.

If the film is above the glass transition temperature its glassy stateis unstable; close to the transition temperature the rate ofcrystallisation is slow but large numbers of nuclei are formed. It ispreferred that the surface to which the layer is applied should be at atemperature between the glass transition temperature and 30° C. and morepreferably at most 20° C. and that the film should be raised to atemperature of 45° to 100° C. for example 50° to 90° C. and preferably60° to 80° C. to permit crystallite growth.

The polymer may be or include at least one polyester having units offormula I: ##STR1## wherein m is in the range 1 to 11 and n is 2m or(when m is at least 2) 2m-2.

In preferred polyesters, m is 1 or 2, n is 2m and there may be unitswith m=1 and m=2 copolymerised together. Certain preferred polyesterscontain a preponderance of m=1 units, especially with 70 to 98 mol % ofsuch units, the balance being units in which m=2. The molecular weightof the polymer is preferably over 100,000, particularly for supportedfilms, and more preferably over 300,000, particularly for unsupportedfilms.

The polyhydroxyalkanoate is preferred for many applications to be ablend of two or more polymers differing in the value of m. A particularexample contains:

a) a polymer consisting essentially of formula I units in which 2 to 5mol % of units have m=2 and the rest m=1; and

b) a polymer consisting essentially of formula I units in which 5 to 30mol % of units have m=2, the rest m=1.

The proportions of the polymer in such a blend is preferably such as togive an average m=2 content in the range 2 to 28 mol %, and typically 4to 18 mol %. In its preferred form the polymer can therefore be seen tobe a poly{hydroxybutyrate/valerate} copolymer or blend of copolymers.

The polyhydroxyalkanoate may be a fermentation product, particularly ofa microbiological process, whereby a microorganism lays downpolyhydroxyalkanoate during normal or manipulated growth. Manipulationmay be achieved by removing or failing to provide one or more nutrientsnecessary for cell multiplication. The microorganisms may be wild typeor mutated or may have the necessary genetic material introduced intoit, for example by any of the methods of recombinant DNA technology. Itis to be emphasised that it is not necessary for thepolyhydroxyalkanoate-producing organism to be a microorganism, but atpresent such organisms are preferred.

A number of suitable microbiological processes for the production ofpolyhydroxyalkanoate polymers have been described in the art. Forexample, to produce formula I material with m=1 or m=partly 1, partly 2,the process of EP-A-0069497 may be used using the species Alcaligeneseutrophus. For formula I materials with m=1, the process of U.S. Pat.No. 4,101,533 can be used; this process uses the microorganismAlcaligenes eutropha H-16. Similar materials can be produced usingAlcaligenes latus, as disclosed in EP-A-0144017. For formula I materialswith m having a value from 3 to 7 various Pseudomonas spp. can be used,as disclosed in EP-A-0392687.

In the process of the invention, the film is formed by applying thelayer of the molten polyhydroxyalkanoate polymer to a surface. Thisapplication can be achieved by melt extrusion through an appropriatelyconfigured melt extrusion die. The configuration of the die is notbelieved to be particularly critical, and the precedents set in lowdensity polyethylene melt extrusion methodology can be followed orreadily adapted by those skilled in the art.

Molten polyhydroxyalkanoate may be co-extruded with one or more otherpolymers, which may have additional useful functionality (for example,by functioning as adhesives or barrier layers, for example). Generally,the polyhydroxyalkanoate will be the layer which comes into contact withthe surface of the chill-roll.

By means of the invention, films may be formed onto a web, which therebybecomes coated. The web may be any suitable material, such as paper orfabric, which may be knitted, woven or even non-woven. In the case of anon-woven fabric, the melt may penetrate into the non-woven structure,following the contours of the fabric. The web could also be a film of asecond polymer. The second polymer should be a compatible polymer if amultilayer film is required. Alternatively, the second polymer should benon-compatible if it is to be stripped from the first; this may leavethe polyhydroxyalkanoate film with qualities superior to a comparablecast film.

A web may, prior to being coated, be pre-treated for example by a coronatreater. In practice, any suitable pre-treatment can be used ifpretreatment is required at all.

The invention is suitable for preparing cast films, in which thepolyhydroxyalkanoate film is not supported by a web. The film may becast either onto a roller, as in certain chill-roll processes known frompolyethylene casting, or the invention may use an adaption of such aprocess, for example using an endless belt instead of a roller. The castfilm may then be stretched, if required, before or while heating of thefilm takes place.

The temperature of the chill-roll or other surface to which the polymeris cast may be controlled by any suitable refrigeration means. A heattransfer fluid such as water may, when appropriately remotely chilled,be used in an indirect refrigeration system.

The temperature of the surface used in the invention will depend on theglass transition temperature of the polymer from which film is beingformed, which in turn varies with the polymer composition. For apolyhydroxybutyrate/valerate copolymer, the glass transition temperaturedecreases as the hydroxyvalerate content increases. The glass transitiontemperatures for polymers suitable for use in the invention can readilybe determined by those skilled in the art; typically, many glasstransition temperatures of suitable polymers will be in the region of-10° C. to 20° C., the exact value depending on the technique used inits determination. Table 1 shows typical values derived by DynamicMechanical Analysis, as one example of an analytical approach.

                  TABLE 1                                                         ______________________________________                                        Comonomer HV Level (%)                                                                          *Glass Transition (°C.)                              ______________________________________                                         0                10                                                           3                8                                                            9                6                                                           14                4                                                           20                -1                                                          25                -6                                                          ______________________________________                                         *position of E" peak at 5 Hz                                             

When the film is being formed on the surface, the polymer (and/orsurface) temperature should not be substantially above the glasstransition temperature, in accordance with the invention. Thetemperature should therefore be below, at or near the glass transitiontemperature. If the temperature is above the glass transitiontemperature, it will as a rule not be more than about 15° or 20° C.above the glass transition temperature of the base, pre-compoundedpolymer, as measured by Dynamic Mechanical Thermal Analysis (DMTA). Inpractice, the temperatures used will generally be below about 30° C., soas to be sufficiently close to, or below, the glass transitiontemperature of the polymer. However, there is an advantage in not havingthe temperature too low, particularly in humid conditions, as waterdroplets will form below the dew point.

At the temperature of the chill-roll or other surface used for filmformation in the invention, relatively little or no crystallisation willoccur at the temperatures used. In this way, the film will be in aglassy state. The number of nucleation sites formed at thesetemperatures will be significantly higher than if the film had been castonto a chill-roll held at the optimum temperature for the rate ofcrystallisation of the polymer, which is typically 40° to 60° C. higher.The overall crystallisation rate may be faster due to the increasednumber of nucleation sites, and hence higher line speeds may beachieved.

After the film has been formed, its temperature is raised, in accordancewith the invention, towards (and preferably so as to be at or near) theoptimum temperature for growth of the polymer's spherulites. Thecrystallisation temperature is above the glass transition temperatureand may vary from polymer to polymer. In a polyhydroxybutyrate/valeratecopolymer, it may vary with hydroxyvalerate content. If the supported orunsupported film of polymer is formed on a roller, the film willgenerally be removed from the roller before the temperature isincreased.

The film temperature may be increased by passing the film through anoven or past one of more infra-red heaters, for example. It is to bestressed that any suitable method of heating can be used. Thetemperature of the film will generally be brought into the range of from40° to 100° C. typically about 50° to 90° C. The dwell time of the filmin the heated area will generally be sufficient to cause crystallisationof: a sufficient proportion of the polymer. The dwell time used inpractice is chosen to give adequate crystallite growth rate for thepolymer used and will often be from 1 second to 10 seconds, typicallyabout 1.5 to 3 seconds.

The presence of a nucleating agent is often preferable in the polymercomposition. Examples of conventional nucleating agents (to which theinvention is not limited) include boron nitride, (see EP-A-0291024),ammonium chloride (see WO-A-9119759) and DZB/ZnSt (see EP-A-0400855).The amount of nucleating agent if present will generally be less than orequal to one part per hundred resin (phr) if only because of the wastedcost of adding excess nucleating agent. The lower limit of nucleatingagent present is effectively 0 phr, which is to say that its presence isoptional.

Another optional ingredient in the polymer composition is a releaseagent. Conventional release agents may be used, but the invention is notin any way restricted to the use of release agents which are known todate.

A further optional ingredient is a plasticiser. Plasticisers may improvethe processability of the composition or improve its mechanicalperformance. Again, conventional plasticisers may be used.

Films, whether coated on a web or free-standing, of various thicknessescan be produced by means of the invention. In particular, films of from1 to 100 μm, and typically from 5 to 40 μm, may be produced.

According to a second aspect of the invention, there is provided a filmproduced by the above process. Preferred features for the second aspectof the invention are as for the first aspect, mutatis mutandis.

This invention also comprises a polymer of one or more hydroxyalkanoicacids which may be as hereinafter described of which at least one ishydroxybutyric acid of which 40 to 70% is crystalline (as judged byX-ray diffraction) and of which at least 80% of the crystallites ofdiameter greater than 3 μm are smaller than 7 μm in diameter (thediameter being calculated as that of a sphere of volume equal to thecrystallite) and films made therefrom.

For a better understanding of the present invention, and to show how itmay be put into effect, reference will now be made by way of example tothe accompanying drawing, in which:

FIG. 1 shows, schematically and not to scale, an apparatus suitable forfilm-coating a web in accordance with the invention.

Referring now to FIG. 1, a web-coating apparatus 1 comprises achill-roll 3 flanked each side by one of a pair of nip-rolls 5 and 7each of which cooperates with the chill-roll and rotates in an oppositesense to it. A web 9 of paper or other suitable material enters thedevice from the right in FIG. 1 and passes through a corona treater 17,which may be used to pretreat the surface of the paper prior to beingcoated by the polymer. The web then passes over the first nip-roll 5. Asthe web passes into the nip between the first nip-roll 5 and thechill-roll 3, molten polyhydroxyalkanoate is applied to the nip from anextruder 11 positioned above it for that purpose. The now coated web 9passes round the under side of the chill roll, with thepolyhydroxyalkanoate polymer remaining in contact with it and becomingsufficiently solid or glassy by virtue of the lower temperature of thechill-roll 3. The coated web 9 now passes through and emerges from thenip formed between the chill-roll 3 and the second nip-roll 7,whereafter it passes underneath an infra-red heater 19, set such thatthe temperature so that the temperature of the polymer rises to ortowards its optimum temperature for spherulite growth rate. The web 9with now-crystalline polyhydroxyalkanoate polymer coated on it emergesfrom underneath the heater, passes over a free moving roller 21 andmoves through to a wind-up roll 27, where it is stored temporarily.

The invention will now be illustrated with the following examples.

EXAMPLE 1

A formulation of polyhydroxybutyrate/valerate copolymer was prepared,having a hydroxyvalerate content of 10 mol % and 1 phr boron nitridenucleating agent. Granules of the formulation were fed into an extruderwith a 40 mm diameter screw. The screw was operated at a speed of 30 rpmand the associated current draw was 16A. The extruder fed into a 20 cmwide die. The temperature of the melt measured close to the die was 180°C. according to a calibrated thermocouple.

In the apparatus shown in FIG. 1, molten polymer was extruded from theextruder 11 into the nip between the first nip-roll 5 and the chill-roll3. The chill-roll was water cooled to 4° C. The gap between the die lipswas set to 0.4 mm; the air gap between these lips and the nip was set toabout 10 cm. In this Example, the polymer was coated on the paper at aline speed of 60 m/min. Downstream of the chill-roll, the coated web washeated to approximately 80° C. with an infra red heater so as tocrystallise the polymer. A 10 μm coating was produced on the paper. Thecoating was satisfactory in its appearance and ran smoothly through tothe wind-up. The coated paper could be unwound without any difficulties.

EXAMPLE 2

The procedure of Example 1 was followed, except that the formulation hada 6 mol % hydroxyvalerate content. The boron nitride content was still 1phr. The melt temperature was 193° C., the screw-speed of the extruderwas 30 rpm and the current draw 20A. In this case, the chill-roll wasset at 12° C. but the gap between the lips of the die was 0.3 mm and theair gap between the die and the nip was set to about 5 cm. An entirelysatisfactory coating (thickness not measured) onto paper was produced ata line speed of 40 m/min.

EXAMPLE 3

The procedure of Example 2 was followed, except that the melttemperature was 190° C. and the chill-roll was set at 8° C. Again, anentirely satisfactory coating was produced on paper at a line speed of40 m/min.

Thin cross sections of the film and coatings according to the inventionmay be tested for crystallite size as follows. They are microtomed andexamined by transmitted light microscopy. They are examined in crossedpolars to reveal the crystalline texture of the samples.

For coatings produced by the method in this invention fine spherulitestructure is observed. The size of the spherulites and their structuredepends upon the exact formulation used. In general however, at least80% of the spherulites observed of diameter greater than 3 μm are <7 μmin diameter and many in the range 3-5 μm.

Film according to the invention is shown to have good barrier propertiesby the following tests. For each of the coatings, an iodine or dyedwater solution was wiped on the surface of the coating. No or fewpinholes were observed as a result of the test. Scanning electronmicrocopy indicated little voiding at the surface of the coating.

I claim:
 1. A process for preparing a film of a polyhydroxyalkanoatepolymer, the process comprising applying a layer of moltenpolyhydroxyalkanoate to a surface not substantially above the glasstransition temperature of the polymer so as to form a film andsubsequently raising the temperature of the film so formed towards theoptimum temperature for growth of the polymer's crystallites.
 2. Aprocess in which a film of a hydroxyalkanoate polymer is prepared whichcomprises applying molten polyhydroxyalkanoate to a surface which isbetween the glass transition temperature of the polyhydroxyalkanoate and30° C. so as to form a film and subsequently substantially increasingits crystallinity by raising the film to a temperature of 45° C. to 100°C.
 3. A process as claimed in claim 2, wherein the polymer is orincludes at least one polyester having units of formula I: ##STR2##wherein m is in the range 1 to 11 and n is 2m or (when m is at least 2)2m-2.
 4. A process as claimed in claim
 3. wherein the polymer is apoly{hydroxybutyrate/valerate} copolymer or blend of copolymers.
 5. Aprocess as claimed in claim 1, wherein the temperature of the saidsurface is from 4° to 20° C.
 6. A process as claimed claim 1, whereinthe temperature of the film formed in the film formation step isincreased so as to be at or near the optimum temperature for growth ofthe polymer's crystallites.
 7. A process as claimed claim 1, wherein thetemperature of the film is brought into the range of from 40° to 100° C.8. A process as claimed in claim 2, wherein a nucleating agent isprovided in association with the molten polyhydroxyalkanoate.
 9. Aprocess as claimed in claim 2, wherein a release agent is provided inassociation with the molten polyhydroxyalkanoate.
 10. A process asclaimed in claim 2, wherein a plasticiser is provided in associationwith the molten polyhydroxyalkanoate.
 11. A process as claimed in claim2, wherein the film has a thickness of from 1 to 100 μm.